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The blood type diet is a nutritional diet advocated by Peter J. D'Adamo, outlined in his book Eat Right 4 Your Type. D'Adamo claims that ABO blood type is the most important factor in determining a healthy diet and recommends distinct diets for each blood type.
Throughout his books, D'Adamo cites the works of biochemists and glycobiologists who have researched blood groups, claiming or implying that their research supports this theory. The consensus among dietitians, physicians, and scientists is that the theory is unsupported by scientific evidence.        
D'Adamo's premise is that blood type is key to the human body's ability to differentiate self from non-self. Lectins in foods, he asserts, react differently with each ABO blood type and, to a lesser extent, with an individual's secretor status. In "Lectins: The Diet Connection" and subsequent chapters of Eat Right 4 Your Type, lectins which interact with the different ABO type antigens are described as incompatible and harmful, and that the selection of different foods for A, AB, B, and O types is therefore important in minimizing reactions with these lectins.
D'Adamo bases his ideas on the ABO classification system of Karl Landsteiner and Jan Janský, as well as some of the many other tissue surface antigens and classification systems, particularly the Lewis antigen system for ABH secretor status.
On page 20 of Eat Right 4 Your Type:20, D'Adamo states, "At this point, you might be wondering about other blood type identifiers, such as positive/negative, or secretor/non-secretor. ... These variations or subgroups within blood types play relatively insignificant roles. More than 90% of the factors associated with your blood type are related to your primary blood type, O, A, B, or AB."
The evolutionary theory of blood groups used by D'Adamo stems from work by William C. Boyd, an immunochemist and blood type anthropologist who made a worldwide survey of the distribution of blood groups. In his 1950 book Genetics and the races of man: An introduction to modern physical anthropology, Boyd describes how through genetic analysis of blood groups, modern humans can be categorized into populations that differ according to their alleles. Boyd divided the world population into 13 geographically distinct species with slightly different frequency distributions of blood group genes.
D'Adamo groups those thirteen races together by ABO blood group, each type within this group having unique dietary recommendations:
The blood type diet has met with criticisms for many different reasons, some of which have been addressed publicly by D'Adamo. A recent scientific review concludes that no scientific evidence exists to support the blood type diet theory and calls for properly designed scientific studies to address it: "No evidence currently exists to validate the purported health benefits of blood type diets. To validate these claims, studies are required that compare the health outcomes between participants adhering to a particular blood type diet (experimental group) and participants continuing a standard diet (control group) within a particular blood type population."
One criticism is that he provided inadequate evidence; his first book, Eat Right 4 Your Type:371 contains only a bibliography. Although his subsequent books have provided thorough references for the classifications of various foods within his categories of "beneficials", "neutrals", and "avoids", his specific process and reasons for reaching these conclusions of classification remain undocumented.
By restricting the complex processes of the human body to just four limiting stereotypes, the blood type diet has been described as "blood type astrology".
There is a lack of scientific evidence supporting the associations between disease states and ABO blood types as mentioned on Peter D'Adamo's website. A search of PubMed under the author's name does not yield any peer-reviewed articles with data to support his claims. For example, his claim that elderberry can be used as a remedy for the common flu lacks scientific evidence and may be misleading. A review article by Dr. Guo and colleagues, published in the American Journal of Medicine, reports that "the effectiveness of any complementary and alternative therapy for treating or preventing seasonal influenza is not established beyond reasonable doubt. Current evidence from randomized controlled trials is sparse and limited by small sample sizes, low methodological quality, or clinically irrelevant effect sizes."
D'Adamo claims many ABO specific lectins exist in foods.:23 This claim is unsubstantiated by established biochemical research, which has found no differences in the reactions of lectins with human ABO types. Research shows that lectins specific to a particular ABO type are generally not found in foods (with several rare exceptions, e.g., the Lima bean) and that lectins with ABO specificity are more frequently found in non-food plants or animals. 
A study by The American Journal of Clinical Nutrition that has been cited to support D'Adamo's theories reported that the edible parts of 29 of 88 foods tested, including common salad ingredients, fresh fruits, roasted nuts, and processed cereals, were found to possess significant lectin-like activity (as assessed by hemagglutination and bacterial agglutination assays). However, nearly all of the 29 foods agglutinated red blood cells of all ABO blood types and were not type-specific; D'Adamo's theory refers to lectins in food that react to specific ABO blood types.
D'Adamo has remarked in the past that it is an oversimplification of his work with blood groups to simply apply the lectin-blood group specifics ad hoc to his work, since that "would not be following the Blood Type Diet, but rather a lectin-avoidance diet".[this quote needs a citation] He states that the blood type diet is characterized more by "what you eat rather than what you avoid", and that "the lectin connection was only a part of a much larger picture."[this quote needs a citation]
I am beginning the eighth year of a ten year trial on reproductive cancers, using the Blood Type Diets. My results are encouraging. So far, the women in my trial have double the survival rate published by the American Cancer Society. By the time I release the results in another 2 years, I expect to make it scientifically demonstrable that the Blood Type Diet plays a role in cancer remission.
However, the results of this trial have never been published. In his book Arthritis: Fight It With the Blood type Diet, D'Adamo mentions an impending clinical trial of the blood type diet to determine its effects on the outcomes of patients with rheumatoid arthritis, but the results of this 12 week trial have also never been published. A study published 16 and 7 years, respectively, after the books (in July 2013) turned up no published results of any such trials.
Luiz C. de Mattos and Haroldo W. Moreira point out that D'Adamo's assertion that the O blood type was the first human blood type requires that the O gene have evolved before the A and B genes in the ABO locus; phylogenetic networks of human and non-human ABO alleles show that the A gene was the first to evolve. They argue that it would be extraordinary, evolutionarily, for normal genes (those for types A and B) to have evolved from abnormal genes (for type O).
Yamamoto et al. further note:
Although the O blood type is common in all populations around the world, there is no evidence that the O gene represents the ancestral gene at the ABO locus. Nor is it reasonable to suppose that a defective gene would arise spontaneously and then evolve into normal genes.
Another study from 2004 concluded that "[a]ssuming constancy of evolutionary rate, diversification of the representative alleles of the three human ABO lineages (A101, B101, and O02) was estimated at 4.5 to 6 million years ago"; D'Adamo asserts that the ABO blood types emerged between 30,000 and 1,000 years ago.